1. Field of the Invention
The present invention relates to methods using lasers to produce a single-frequency output with a narrow line width, to use of such lasers in master oscillator/power amplifier configurations, and to methods and systems for laser peening based on the same.
2. Description of Related Art
The use of mechanical shocks to form metals and to improve their surface properties has been realized for ages. In current industrial practice, a peening treatment of metal surfaces is accomplished by using high velocity shot. Treatment improves surface properties and very importantly for many applications, results in a part displaying significantly improved resistance to fatigue and corrosion failure. A wide range of components are shot peened in the aerospace and automotive industries. However, for many applications, shot peening does not provide sufficiently intense or deep treatment or cannot be used because of its detrimental effect on the surface finish.
With the invention of the laser, it was rapidly recognized that the intense shocks required for peening could be achieved by means of a laser-driven tamped plasma. B. P. Fairand, et al., “Laser Shot Induced Microstructural and Mechanical Property Changes in 7075 Aluminum,” Journal of Applied Physics, Vol. 43, No. 9, p. 3893, September 1972. Typically, a plasma shock of 10 kB to 30 kB is generated at metal surfaces by means of high energy density (about 200 j/cm2), short pulse length (about 30 nanoseconds) lasers. A thin layer of black paint or other absorbing material on the metal surface provides an absorber to prevent ablation of the metal. A confining or tamping material such as water covers the surface layer providing an increased intensity shock. These shocks have been shown to impart compressive stresses, deeper and more intense, than standard shot peening. In testing, this treatment has been shown to be superior for strengthening components from fatigue and corrosion failure. However, lasers with both sufficient energy and sufficient repetition rate to achieve production throughput at affordable costs have been difficult to provide.
One laser system which has been utilized for this purpose is described in our prior U.S. Pat. No. 5,239,408, entitled HIGH POWER, HIGH BEAM QUALITY REGENERATIVE AMPLIFIER. The laser system described in the just cited '408 patent comprises a high power amplifier in a master oscillator/power amplifier MOPA configuration capable of producing output pulses greater than 20 joules per pulse with the pulse width on the order of 30 nanoseconds or less. The '408 patent refers to U.S. Pat. No. 5,022,033, entitled RING LASER HAVING AN OUTPUT AT A SINGLE FREQUENCY, as one implementation of a master oscillator. The oscillator geometry described in U.S. Pat. No. 5,022,033 produces very low energy pulses and therefore requires many more amplifier passes than is achievable with the amplifier system described in U.S. Pat. No. 5,239,408. In some applications, the master oscillator used in the system of the '408 patent was a standing-wave (2 mirror linear resonator) oscillator with an etalon output coupler.
The performance of the MOPA configuration is limited to a degree by the quality of the seed pulse provided by the master oscillator. The master oscillators of the prior art have been able to supply high-quality single-frequency Q-switched oscillator seed pulses. However, it has been difficult to maintain the pulse width and pulse energy substantially constant in a production environment at a sufficient pulse energies.
In general, it is desirable in many applications to have single-frequency Q-switched oscillator pulses from a solid-state laser, both for generating reproducible smooth temporal profiles, without modulation from multiple longitudinal modes, and for achieving the best wavefront reversal fidelity with amplifiers using SBS conjugation. Single longitudinal mode output from a Q-switched laser oscillator has been demonstrated by injection locking a low-power single-frequency master oscillator. Hanna, et al., “Single Longitudinal Mode Selection of High Power Actively Q-Switched Laser,” OPTICAL-ELECTRONICS 4,239–256 (1972). However, this technique requires careful mode matching between the master and slave oscillators and active cavity length stabilization for the Q-switched slave oscillator.
Another method for achieving single frequency output is provided as described in the U.S. Pat. No. 5,022,033, using a self-seeded resonator. A seed pulse was allowed to build up in this prior art approach by maintaining a weak single-frequency, continuous wave (CW) beam in the resonator by managing intracavity loss. An output pulse having beam qualities similar to those of the CW beam is generated by Q-switching because the oscillation which produces the Q-switched pulse builds up from the weak CW beam well ahead of any competing mode. However, the energy per pulse and pulse widths can vary with for example drift in the pump energy, supplied by flashlamps or other pump energy sources, for the ring laser. Thus, the consistency of pulse parameters is relatively poor, and for laser peening operations with output pulses as consistent as possible, it would be necessary to constantly adjust optical parameters of the seed oscillator, such as flashlamp energy and the like. One primary disadvantage of this approach is the long pulse durations (200 ns) and low pulse energies that result from the inefficiencies of keeping a constant, CW seed oscillation running between pulses.
A variation of a self-seeded oscillator is described in Park et al., “Electronic Line WidTh Narrowing Method for Single Axial Mode Operation of Q-Switched Nd:YAG Lasers,” OPTICAL COMMUNICATIONS 37,411–416 (1981).
Thus, it is desirable to provide sequences of pulses, used as seed pulses for a laser peening MOPA for example, with substantially constant energy, substantially constant pulse width, and at a single frequency in sequences of pulses over time intervals that are relevant in a production environment for laser peening and other applications.